the machine is dedicated to Zimbra entrirely. However if it only leaves 65 megs during "no traffic" how much will it eat when it goes live? The only traffic it gets currently is uploading the messages from the old zimbra foss server.

The FOSS machine is running on VMWare as well. THere I gave it 3GB which it leaves usually around 500megs. And that has around 20 users and mails coming in all the time, greylisting, spam...etc

Is this memory difference due to the Network and FOSS differences ? I thought 4GB would be engough for around 20 users right? But with the rate its going I am not sure anymore. It would be good to know if this would improve over time, or should I just buy more memory before putting it into production?

Editing the number of file descriptors available to slapd in 5.0.2 will significantly reduce memory pressure. This fix will be in 5.0.3. To fix this, as root edit /opt/zimbra/libexec/zmslapd so it looks like:

Note: "free" memory is wasted memory in Linux. Any memory not currently and actively being used by programs *will* be used as a disk/data cache. When programs need memory, pages from the cache are freed for them to use.

You only have to worry when SWAP usage starts climbing into the gigabytes. Until then, relax in the knowledge that the kernel is doing it's job managing memory in such a way to reduce the amount of disk traffic.